1. Field of the Invention
This invention relates to A.C. ionizers and more particularly relates to a method and control for balancing the positive and negative ion output of such air ionizers whereby efficient static neutralization of the article surface or zone at which the ionization is directed may be provided.
2. Prior Art
As is well known, static eliminators are devices for producing both positive and negative ions in order to neutralize articles or surfaces which have been charged to a particular polarity. A typical electrical air ionizer operating on alternating electrical current comprises a high voltage transformer whose output is connected to one or more sharp electrodes located in the proximity of electrical ground. A.C. current is used because of the desire to produce both positive and negative ions essentially simultaneously. While both positive and negative ion production could be exactly equal under certain circumstances, in most instances, ions of a particular polarity will predominate depending upon the geometry of the A.C. air ionizer and whether the ionizing points are directly or capacitively coupled to the A.C. high voltage.
The ions of either polarity are only created or generated when the voltage on the discharge electrodes exceeds the corona onset threshold or level. That is, there is no ion generation when the voltage on the points is below the corona threshold. This corona onset level is a function of the sharpness of the discharge electrodes, its distance from a proximity ground and certain other variables, for example, atmosphere of operation and electrode contamination.
A schematic representation of a direct coupled air ionizer is shown in FIG. 1 wherein the high side of a transformer T is connected directly to points P which are adjacently spaced from a proximity ground, usually in the form of a conductive casing A. However, the corona electrode P to which an A.C. high voltage is applied can be shown as having an equivalent circuit, as set forth in FIG. 2 wherein:
Capacitance C2 is the total capacitance of the corona electrode to ground;
Variable resistance R1 is the electrical resistance of the air between the ionizing electrode P and ground when ions are being generated (i.e. electrode voltage above corona onset); and
Switch SW represents the intermittent nature of the ion flow - the switch being open when the point voltage is below corona onset (no ions being generated in the air gap) and closed when the point voltage exceeds the corona onset (ions flowing from point P through the air gap to ground).
In the direct coupled A.C. air ionizer, such as shown in U.S. Pat. No. 3,137,806 or U.S. Pat. No. 3,156,847 (air gun), there is usually a predominance of negative ions emitted even though the discharge points are connected to an A.C. high voltage source having equal positive and negative high voltage amplitudes. This excessive negative ion production is the result of the inherent differences in positive and negative corona characteristics, firstly because the threshold voltage for corona onset in the case of negative corona is lower than for positive thereby causing a longer period during which the negative ions are created, and secondly because the current versus voltage curve (I-V) is much steeper in the case of negative than with the positive.
This may be readily seen from the curves of FIG. 3 wherein as a result the negative corona is present for a longer period of time than positive corona. Furthermore, the resistance of air between the corona electrode and ground is lower for the negative corona in part because of the greater mobility of negative ions. To reflect these differences, the equivalent circuit in FIG. 4 has been modified, wherein:
R1(-) represents the resistance of air during a negative half-cycle of operating voltage, and
R1(+) represents the resistance during a positive halfcycle of operating voltage.
While the switching action of switch SW should produce a D.C. bias voltage on the point-to-ground capacitance C2 because of the difference in the positive and negative corona onset and corona current, this bias cannot sustained because instantaneous bleed-off to ground occurs through the low impedance of the transformer secondary.
In prior U.S. Pat. to Walkup, No. 2,879,395, a small D.C. power supply was incorporated between the proximity ground casing and the A.C. generator. It functioned by placing a D.C. bias of the proper polarity on the casing or on the discharge points and was connected in such a way as to retard the output of ions of the usually predominant polarity and/or enhance the output of ions of the opposite polarity. However, the additional power supply even though small made this system bulky and expensive.
Another but less expensive system of balancing the production of positive and negative ions is shown in the Takahashi Patent No. 3,714,531 wherein a diode-resistor parallel circuit replaced the small D.C. generator of Walkup. However, this system does not provide balanced ionization under changing voltage or varying environmental conditions, and the resistor has to be readjusted with every change in applied voltage.
In Antonevitch Patent No. 4,423,462, the biasing circuit for balancing was connected to the primary of a transformer and included a series-connected diode and variable resistor in one leg of a parallel circuit and a capacitor in the other. By selecting appropriate time constants for the resistance and capacitance, one could narrow the first half of the sine wave and broaden the second half to produce a desired number of positive or negative ions as desired depending upon the direction of the diode. The emission could be controlled to yield an equal number of ions of each polarity or a predominance of one polarity regardless of whether the A.C. high voltage was directly connected or capacitively coupled to the points. The problem with this design, as with the one before, is that constant readjustment is required as conditions change.
In Levy Patent No. 4,092,543, additional pointed needles were adjustably positioned in adjacently spaced from and interacting disposition with at least some of the discharging electrodes and means constituting a conductive path to the other side of the A.C. supply were provided to draw off a portion of the negative ion emission so as to enable emission of an equal number of ions of each polarity.
In the case of capacitively coupled air ionizers there is usually a predominance of positive ions emitted, the greater positive ion production resulting from the fact that a D.C. voltage is developed across the coupling capacitance in the direction which biases the points slightly in a positive direction. That is, in a capacitively coupled system, such as shown in Schweriner Patent No. 3,120,626 or No. 3,179,849 (air gun), a capacitance was included between the discharge points and the power supply in order to limit the short circuit current that can be drawn from a point.
In U.S. Pat. No. 4,188,530, an air stream was blown across the capacitively coupled points in order to extend the range of ionization. However, while it has been common to utilize capacitive coupling between the high voltage transformer and the ionizing electrodes, as shown in the schematic circuit of FIG. 5, in order to limit current for safety purposes, it has not been understood until now how the capacitive coupling may serve as an automatic compensator for the difference in the positive and negative corona characteristics.